1. Field of the Invention
The present invention relates to a communication system, in which a plurality of mobile stations perform half-duplex many-to-many communication, and to a press-talk server and a transcoder in the communication system. In particular, the present invention relates to a communication system, such as a professional mobile radio system, in which a plurality of mobile stations participating in a call have different codecs, to a transcoder, and to a press-talk server.
2. Description of the Related Art
In public services, a professional mobile radio system provides a half-duplex many-to-many communication (group communication), called press-talk communication. The outline of the press-talk call will now be described.
If a mobile station calls a group identified by a group number, a press-talk server connects a call among a plurality of mobile stations as group members. Each of the plurality of mobile stations participating in the call receives every voice sent from other group members. In order to send voice, a user of a mobile station has to press a button in the mobile station to request the press-talk server for a voice talk burst, and has to be permitted to send a talk burst. The press-talk server performs exclusive talk burst control such that only one mobile station takes a talk burst in the group at a time. A user who requests a talk burst is noticed the permission of the talk burst by a buzzer or lamp lighting and sends voice. The press-talk server delivers the sent voice to other group members. The user who sent voice releases the pressed button when the user finished talking. A talk burst release indication is received by the press-talk server, and the press-talk server waits for a next talk burst request. If a caller requests call disconnection, the call is terminated.
In general, mobile stations use the same codec in a system. For this reason, in case of communication between the mobile stations, transcoding is not required. However, a terminal, such as a dispatch console, which is connected to the press-talk server in a wired manner, uses an ITU-T G.711 codec different from the codec of the mobile stations. For this reason, in case of communication between a mobile station and a wired terminal, transcoding is required. Even in case of communication between the mobile stations, when the mobile stations belong to different systems or when codecs to be used are different, transcoding is required.
The reason why the mobile station and the wired terminal use different codecs is as follows. That is, while the mobile station uses a codec of a low bit rate such that voice information may be transmitted even in a narrow radio band, the wired terminal requires a function to mix voice of a plurality of press-talk calls and simultaneously output the mixed voice, and as a result uses a PCM-based codec such that PCM (Pulse-Code Modulation) as a voice signal after being mixed may be reproduced without deteriorating voice quality.
Transcoding for communication between the mobile station and the wired terminal is performed on a base station line interface card in the press-talk server. In the base station line interface card, a codec circuit exists for a mobile station which performs communication through the base station line. Voice from the mobile station is transcoded by the codec circuit for the mobile station, and is sent to the wired terminal. Voice from the wired terminal is transcoded by the codec circuit, and is sent to the mobile station through a channel and the base station.
As another technique, a PoC (Push-to-talk over Cellular) for a third generation mobile phone service is known. The PoC implements a press-talk call among mobile phones using the same codec over IP. Typically, a press-talk call among mobile stations using different codecs is impossible and nor required. The reason is as follows. That is, the 3GPP (The 3rd Generation Partnership Project) and the 3GPP2 are unifying the codecs of the mobile stations, and each communication carrier provides PoC service only to its subscribers whose terminals conform to 3GPP or 3GPP2 standards. Furthermore, a mixing service like that of a wired terminal in a professional mobile radio system is not required.
As yet another technique, a media server for a conference call over IP telephone is known. A conference call over IP telephone is a full-duplex many-to-many call. If a conference call is generated, the media server simultaneously receives voice packets from all terminals participating in the call, and classifies the voice packets in a dejitter buffer (hereinafter, simply referred to as a jitter buffer) of each terminal. Voice of each terminal is converted to PCM by a codec circuit allocated to the jitter buffer, and mixed. The mixed voice is converted by the codec of the terminal again, and is sent to the terminal. The codec circuit allocated to the jitter buffer is decided per call.
Non-patent Document 1: “System Development for Push Talk Service” NTT DoCoMo Technical Journal, Vol. 13, No. 4 is an example of related art.